Two doors which move independent of one another is the makeup of a Dutch Door. While most doors have three hinges, this design requires a fourth. A dutch door essentially combines a door with a large window.
The bottom portion contains the doorknob and lock, and the top portion contains a latch to attach the two portions together. When latched together, a dutch door will preform the same as a traditional door. Often, they have something that resembles a window sill at the top of the bottom portion.